The term "polyphenylene ether resin" is well known as defining a class of thermoplastic materials which possess outstanding physical properties, including hydrolytic stability, dimensional stability and excellent dielectric characteristics. Methods of preparation are known in the art and described in the patent literature, e.g., Hay, U.S. Pat. Nos. 3,306,874, and 3,306,875 and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, incorporated herein by reference to save unnecessary detail. Other patents which show the preparation of polyphenylene ether resins include Bennett and Cooper, U.S. Pat. Nos. 3,369,656 and 3,838,102, as well as Cooper and Bennett, U.S. Pat. Nos. 3,642,699, 3,661,849 and 3,733,299, also incorporated herein by reference.
It is known that when the polyphenylene ether resins are combined with polystyrene, the resulting compositions possess properties which are improved over the corresponding properties of either resin alone. See Cizek, U.S. Pat. Nos. 3,383,435, the disclosure of which is incorporated herein by reference.
A shortcoming of these thermoplastic materials is their relatively poor resistance to electrical arcing. Such arcing is most conveniently quantized by the related parameter of tracking (often referred to as "arc tracking") resistance. It measures, for example, the ability of the thermoplastic composition to withstand extreme electrical stresses through the rate of formation of a carbonized path or track as two charged electrodes are drawn apart on its surface. Poor arc tracking resistance is especially undesirable for thermoplastic materials employed as electronic equipment components and housings.
It is known that the arc/tracking resistance of certain polymeric compositions may be improved through the incorporation of mineral fillers. It has recently been proposed that talc, in particular, when employed as a filler for a polyphenylene ether composition provides considerable improvement in the arc tracking rate. This development is described in application Ser. No. 304,947 filed Sept. 23, 1981, assigned to the same assignee as herein.
Mineral fillers in general decrease the impact strength of polyphenylene ether compositions, however. This is a significant problem in the case of recently developed arc track resistant materials such as those referred to above because, in addition to increased arc tracking, good impact strength is also a desirable property for the compositions.